Today, you landed in the country which lies closest to ― and has the most to lose from ― the biggest foreign policy conundrum facing your presidency: North Korea and its strategic arms programs. You need to bring cool, clear, focused, principled and possibly ruthless thinking to bear on this issue.

Above all:"What is your red line?"

The stakes are high. You like winners, and South Korea is arguably the greatest national success story of the 20th century. Assuming a zero-to-hero trajectory, this nation mastered global commerce and adopted democratic governance. These are empowering forces that South Korea shares with America.

Your country's long-term commitment to Korea should be seen, alongside the examples of West Germany and Japan, as one of the finest outcomes of American national mentor-ship. (Regrettably, in more recent decades and years ― in South Vietnam, Iraq, Libya and Afghanistan - American commitment to nation building has wavered, with woeful results. But I digress.)

South Korea lives in the threatening shadow of North Korea. The latter country is hell-bent on developing atomic arms, and the missiles to deliver them to your breakfast table. Your predecessors failed to halt these programs. The weapons they deployed against them ― trade sanctions, diplomatic pressure and (more recently) supply chain sabotage and cyber strikes ― plainly failed.

Before entering the White House, you announced that Pyongyang's possession of nuclear-tipped ICBMs was"Not going to happen." In fact, most indicators are that it is going to happen ― on your watch.

North Korea is winning. It possesses both uranium- and plutonium-based arms programs. It has probably compressed these materials to warhead size. It is believed to have enough for between a dozen and 60 warheads. It has a missile able to cross the Pacific.

All it lacks is a re-entry vehicle to bring that missile back into the atmosphere, and the targeting devices to land it in downtown LA. Rest assured: If not stopped, it will develop these last components.

So, at what point in their program do you say,"That's the red line. If they cross it, we take the kind of actions that we have never taken before."

I cannot say what actions they might be. Direct, high-level negotiations with everything on the table? A campaign of asymmetric or informational warfare? Deniable, "black" operations? Military strikes?

Whatever action you may take ― and you are obliged by treaty to at least discuss military measures with Seoul ― South Korea needs to know the red line that would trigger such action. President Moon insists there can be no war on the Korean peninsula, under any circumstance. If you believe Pyongyang's threat to America is so great that it outweighs South Korea's security, you may differ. Result? The North Korean nuclear crisis could shatter the ROK-US alliance and propel South Korea into China's arms.

The world wants to know your red line as it is in the public interest of everyone from financiers and diplomats to businesspeople and air travelers. (But we don't expect to learn it: Obviously, you don't want to back yourself into a corner.)

Even so, you must know it, because you need focused policy, anchored in principle. Moreover, any assets you deploy if the North does cross your red line ― be they diplomatic, military or other ― need to prepare for worst-case scenarios.

So what is it? Are you willing to let Pyongyang finalize an inter-continental nuclear strike capability?

Perhaps: Few persons worldwide would consider that a casus belli for possible Armageddon; containment and deterrence have decent records; and it seems unlikely that Pyongyang would launch a first strike that would result in retaliation and annihilation.

Perhaps not: You may have privileged information that shows that the risks for America are simply too great to tolerate Pyongyang's possession of doomsday weaponry. Then, at what stage do you assert, "Enough is enough?"

Mr. President, you are a negotiator. Every negotiator has a bottom line. When it comes to Nork nukes, the time remaining for "strategic patience" is almost up. There can be no fuzziness. You need a plain, clear red line.

Andrew Salmon (andrewcsalmon@yahoo.co.uk) is a Seoul-based reporter and author.